I agree - I don't send mine private to obtain an academic advantage.
I come from a v WC background, went to a state primary in a v deprived area, followed, fortunately, by state grammar school. I now work in a good job earning a v good salary.
So when my eldest started school I sent her to a state primary. However, she was repeatedly sent home because she was so anxious that she was making herself sick and then she wouldn't be allowed back in for 48hrs. She was missing so much schooling and whenever I asked what she was missing I was told it wasn't my concern and they would make sure she didn't miss out. She never had sound cards or books sent home because I was told she was up to the requisite standard (albeit she's not particularly academic). I couldn't believe that they wouldn't want to utilise an engaged parent and would prefer to block them out.
So we made the decision to move her to a private school, because I'm earning well so why on earth would I leave her in that situation when I had an alternative. Since moving her she hasn't made herself sick once and she's thriving (not just in an academic way but in her confidence and happiness). I repeatedly flagged her anxiety to her first school, and they refused to acknowledge it but within 2 weeks of moving to the private school they rang me to discuss what they'd observed and implement a plan.
It's absolutely awful that there will be children who don't have this opportunity but, for me personally, why do I work so hard if I can't do something like this to help them. The reality is, if we don't send them to a private secondary, we will just move into the catchment of the excellent grammar - but again, not because of the academic results, but because of the whole environment. And yes, I've heard parents discussing buying a second home near that school and moving in for 6 months just to get a place - so it's foolish to think that MC parents these days won't do whatever it takes.